This invention relates to methods and apparatus for the selection, identification, characterization, and/or sorting of materials utilizing at least optical or photonic forces. More particularly, the inventions find utility in biological systems, generally considered to be the use of optical forces for interaction with bioparticles having an optical dielectric constant.
Separation and characterization of particles has a wide variety of applications ranging from industrial applications, to biological applications, to environmental applications. For example, in the field of biology, the separation of cells has numerous applications in medicine and biotechnology. Historically, sorting technologies focused on gross physical characteristics, such as particle size or density, or to utilize some affinity interaction, such as receptor-ligand interactions or reactions with immunologic targets.
Electromagnetic response properties of materials have been utilized for particle sorting and characterization. For example, dielectrophoretic separators utilize non-uniform DC or AC electric fields for separation of particles. See, e.g., U.S. Pat. No. 5,814,200, Pethig et al., entitled xe2x80x9cApparatus for Separating By Dielectrophoresisxe2x80x9d. The application of dielectrophoresis to cell sorting has been attempted. In Becker (with Gascoyne) et al., PNAS USA, Vol. 92, pp. 860-864, January 1995, Cell Biology, in the article entitled xe2x80x9cSeparation of Human Breast Cancer Cells from Blood by Differential Dielectric Affinityxe2x80x9d, the authors reported that the dielectric properties of diseased cells differed sufficiently to enable separation of the cancer cells from normal blood cells. The system balanced hydrodynamic and dielectrophoretic forces acting on cells within a dielectric affinity column containing a microelectrode array. More sophisticated separation systems have been implemented. See, e.g., Cheng, et al., U.S. Pat. No. 6,071,394, xe2x80x9cChannel-Less Separation of Bioparticles on a Bioelectronic Chip by Dielectrophoresisxe2x80x9d. Yet others have attempted to use electrostatic forces for separation of particles. See, e.g., Judy et al., U.S. Pat. No. 4,440,638, entitled xe2x80x9cSurface Field-Effect Device for Manipulation of Charged Speciesxe2x80x9d, and Washizu xe2x80x9cElectrostatic Manipulation of Biological Objectsxe2x80x9d, Journal of Electrostatics, Vol. 25, No. 1, June 1990, pp. 109-103.
Light has been used to sort and trap particles. One of the earliest workers in the field was Arthur Ashkin at Bell Laboratories, who used a laser for manipulating transparent, xcexcm-size latex beads. Ashkin""s U.S. Pat. No. 3,808,550 entitled xe2x80x9cApparatuses for Trapping and Accelerating Neutral Particlesxe2x80x9d disclosed systems for trapping or containing particles through radiation pressure. Lasers generating coherent optical radiation were the preferred source of optical pressure. The use of optical radiation to trap small particles grew within the Ashkin Bell Labs group to the point that ultimately the Nobel Prize was awarded to researchers from that lab, including Steven Chu. See, e.g., Chu, S., xe2x80x9cLaser Trapping of Neutral Particlesxe2x80x9d, Sci. Am., p. 71 (February 1992), Chu, S., xe2x80x9cLaser Manipulation of Atoms and Particlesxe2x80x9d, Science 253, pp. 861-866 (1991).
Generally, the interaction of a focused beam of light with dielectric particles or matter falls into the broad categories of a gradient force and a scattering force. The gradient force tends to pull materials with higher relative dielectric constants toward the areas of highest intensity in the focused beam of light. The scattering force is the result of momentum transfer from the beam of light to the material, and is generally in the same direction as the beam. The use of light to trap particles is also sometimes referred to as an optical tweezer arrangement. Generally, utilizing the Rayleigh approximation, the force of trapping is given by the following equation:       F    g    =      2    ⁢          π      ·              r        3              ⁢                            ϵ                B            c        ⁢          (                        ϵ          -                      ϵ            B                                    ϵ          +                      2            ⁢                          ϵ              B                                          )        ⁢          (              ∇                  ·          I                    )      
where Fg is the optical gradient force on the particle in the direction toward the higher intensity, r is the radius of the particle, ∈B is the dielectric constant of the background medium, ∈ is the dielectric constant of the particle, I is the light intensity in watts per square centimeter and ∇ is the spatial derivative. FIG. 1 shows a drawing of a particle in an optical tweezer. The optical tweezer consists of a highly focused beam directed to the particle.
As shown in FIG. 1, the focused beam 12 first converges on the particle 10 and then diverges. The intensity pattern 14 relates to the cross-section of the intensity of the beam in the horizontal dimension, and the intensity pattern 16 is the cross-section of intensity in the vertical dimension. As can be seen from the equation, the trapping force is a function of the gradient of the intensity of the light. Thus, the force is greater where the light intensity changes most rapidly, and contrarily, is at a minimum where the light intensity is uniform.
Early stable optical traps levitated particles with a vertical laser beam, balancing the upward scattering force against the downward gravitational force. The gradient force of the light served to keep the particle on the optical axis. See, e.g., Ashkin, xe2x80x9cOptical Levitation by Radiation Pressurexe2x80x9d, Appl. Phys. Lett., 19(6), pp. 283-285 (1971). In 1986, Ashkin disclosed a trap based upon a highly focused laser beam, as opposed to light propagating along an axis. The highly focused beam results in a small point in space having an extremely high intensity. The extreme focusing causes a large gradient force to pull the dielectric particle toward that point. Under certain conditions, the gradient force overcomes the scattering force, which would otherwise push the particle in the direction of the light out of the focal point. Typically, to realize such a high level of focusing, the laser beam is directed through a high numerical aperture microscope objective. This arrangement serves to enhance the relative contribution from the high numerical aperture illumination but decreases the effect of the scattering force.
In 1987, Ashkin reported an experimental demonstration of optical trapping and manipulation of biological materials with a single beam gradient force optical trap system. Ashkin, et al., xe2x80x9cOptical Trapping and Manipulation of Viruses and Bacteriaxe2x80x9d, Science, Mar. 20, 1987, Vol. 235, No. 4795, pp. 1517-1520. In U.S. Pat. No. 4,893,886, Ashkin et al., entitled xe2x80x9cNon-Destructive Optical Trap for Biological Particles and Method of Doing Samexe2x80x9d, reported successful trapping of biological particles in a single beam gradient force optical trap utilizing an infrared light source. The use of an infrared laser emitting coherent light in substantially infrared range of wavelengths, there stated to be 0.8 xcexcm to 1.8 xcexcm, was said to permit the biological materials to exhibit normal motility in continued reproductivity even after trapping for several life cycles in a laser power of 160 mW. The term xe2x80x9copticutionxe2x80x9d has become known in the art to refer to optic radiation killing biological materials.
The use of light to investigate biological materials has been utilized by a number of researchers. Internal cell manipulation in plant cells has been demonstrated. Ashkin, et al., PNAS USA, Vol. 86, 7914-7918 (1989). See also, the summary article by Ashkin, A., xe2x80x9cOptical Trapping and Manipulation of Neutral Particles Using Lasersxe2x80x9d, PNAS USA, Vol. 94, pp. 4853-4860, May 1997, Physics. Various mechanical and force measurements have been made including the measurement of torsional compliance of bacterial flagella by twisting a bacterium about a tethered flagellum. Block, S., et al., Nature (London), 338, pp. 514-518 (1989). Micromanipulation of particles has been demonstrated. For example, the use of optical tweezers in combination with a microbeam technique of pulsed laser cutting, sometimes also referred to as laser scissors or scalpel, for cutting moving cells and organelles was demonstrated. Seeger, et al., Cytometry, 12, pp. 497-504 (1991). Optical tweezers and scissors have been used in all-optical in vitro fertilization. Tadir, Y., Human Reproduction, 6, pp. 1011-1016 (1991). Various techniques have included the use of xe2x80x9chandlesxe2x80x9d wherein a structure is attached to a biological material to aid in the trapping. See, e.g., Block, Nature (London), 348, pp. 348-352 (1990).
Various measurements have been made of biological systems utilizing optical trapping and interferometric position monitoring with subnanometer resolution. Svoboda, Nature (London), 365, pp. 721-727 (1993). Yet others have proposed feedback based systems in which a tweezer trap is utilized. Molloy, et al., Biophys. J., 68, pp. 2985-3055 (1995).
A number of workers have sought to distort or stretch biological materials. Ashkin in Nature (London), 330 pp. 769-771 (1987), utilized optical tweezers to distort the shape of red blood cells. Multiple optical tweezers have been utilized to form an assay to measure the shape recovery time of red blood cells. Bronkhorst, Biophys. J., 69, pp. 1666-1673 (1995). Kas, et al., has proposed an xe2x80x9coptical stretcherxe2x80x9d in U.S. Pat. No. 6,067,859 which suggests the use of a tunable laser to trap and deform cells between two counter-propagating beams generated by a laser. The system is utilized to detect single malignant cancer cells. Yet another assay proposed colliding two cells or particles under controlled conditions, termed the OPTCOL for optical collision. See, e.g., Mammer, Chem and Biol., 3, pp. 757,763 (1996).
Yet others have proposed utilizing optical forces to measure a property of an object. See, e.g., Guanming, Lai et al., xe2x80x9cDetermination of Spring Constant of Laser-Trapped Particle by Self-Mining Interferometryxe2x80x9d, Proc. of SPIE, 3921, pp. 197-204 (2000). Yet others have utilized the optical trapping force balanced against a fluidic drag force as a method to calibrate the force of an optical trap. These systems utilize the high degree of dependence on the drag force, particularly Stokes drag force.
Yet others have utilized light intensity patterns for positioning materials. In U.S. Pat. No. 5,245,466, Burnes et al., entitled xe2x80x9cOptical Matterxe2x80x9d, arrays of extended crystalline and non-crystalline structures are created using light beams coupled to microscopic polarizable matter. The polarizable matter adopts the pattern of an applied, patterned light intensity distribution. See also, xe2x80x9cMatter Rides on Ripples of Lightsxe2x80x9d, reporting on the Burns work in New Scientist, Nov. 18, 1989, No. 1691. Yet others have proposed methods for depositing atoms on a substrate utilizing a standing wave optical pattern. The system may be utilized to produce an array of structures by translating the standing wave pattern. See, Celotta et al., U.S. Pat. No. 5,360,764, entitled xe2x80x9cMethod of Fabricating Laser Controlled Nanolithographyxe2x80x9d.
Yet others have attempted to cause motion of particles by utilizing light. With a technique termed by its authors as xe2x80x9cphotophoresisxe2x80x9d, Brian Space, et al., utilized a polarized beam to induce rotary motion in molecules to induce translation of the molecules, the desired goal being to form a concentration gradient of the molecules. The technique preferably utilizes propeller shaped molecules, such that the induced rotary motion of the molecules results in translation.
Various attempts have been made to form microfluidic systems, put to various purposes, such as sample preparation and sorting applications. See, e.g., Ramsey, U.S. Pat. No. 6,033,546, entitled xe2x80x9cApparatus and Method for Performing Microfluidic Manipulations for Chemical Analysis and Synthesisxe2x80x9d. Numerous companies, such as Aclara and Caliper, are attempting to form micro-systems comprising a xe2x80x98lab on a chipxe2x80x99.
Others have attempted to combine microfabricated devices with optical systems. In xe2x80x9cA Microfabricated Device for Sizing and Sorting DNA Moleculesxe2x80x9d, Chou, et al., PNAS USA, Vol. 96, pp. 11-13, January 1999, Applied Physical Sciences, Biophysics, a microfabricated device is described for sizing and sorting microscopic objects based upon a measurement of fluorescent properties. The paper describes a system for determining the length of DNA by measuring the fluorescent properties, including the amount of intercalated fluorescent dye within the DNA. In xe2x80x9cA Microfabricated Fluorescence-Activated Cells Sorterxe2x80x9d, Nature Biotechnology, Vol. 17, November 1999, pp. 1109-1111, a xe2x80x9cTxe2x80x9d microfabricated structure was used for cell sorting. The system utilized a detection window upstream of the xe2x80x9cTxe2x80x9d intersection and based upon the detected property, would sort particles within the system. A forward sorting system switched fluid flow based upon a detected event. In a reverse sorting mode, the fluid flow was set to route all particles to a waste collection, but upon detection of a collectible event, reversed the fluid flow until the particle was detected a second time, after which the particle was collected. Certain of these systems are described in Quake et al., PCT Publication WO 99/61888, entitled xe2x80x9cMicrofabricated Cell Sorterxe2x80x9d.
Yet others have attempted to characterize biological systems based upon measuring various properties, including electromagnetic radiation related properties. Various efforts to explore dielectric properties of materials, especially biological materials, in the microwave range have been made. See, e.g., Larson et al., U.S. Pat. No. 4,247,815, entitled xe2x80x9cMethod and Apparatus for Physiologic Facsimile Imaging of Biologic Targets Based on Complex Permittivity Measurements Using Remote Microwave Interrogationxe2x80x9d, and PCT Publication WO 99/39190, named inventor Hefti, entitled xe2x80x9cMethod and Apparatus for Detecting Molecular Binding Eventsxe2x80x9d.
Despite the substantial effort made in the art, no comprehensive, effective, sensitive and reliable system has been achieved.
The methods and apparatus of this relate generally to the use of light energy to obtain information from, or to apply forces to, particles. The particles may be of any form which have a dielectric constant. The use of light for these beneficial purposes is the field of optophoresis. A particle, such as a cell, will have a Optophoretic constant or signature which is indicative of a state, or permits the selection, sorting, characterization or unique interaction with the particle. In the biological regime, the particles may include cells, organelles, proteins, or any component down to the atomic level. The techniques also apply in the non-biological realm, including when applied to all inorganic matter, metals, semiconductors, insulators, polymers and other inorganic matter.
Considering the biological realm, the cell represents the true point of integration for all genomic information. Accessing and deciphering this information is important to the diagnosis and treatment of disease. Existing technologies cannot efficiently and comprehensively address the enormous complexity of this information. By unlocking the fundamental properties of the cell itself, the methods and apparatus described herein create new parameters for cellular characterization, cellular analysis and cell-based assays.
This technology represents a practical approach to probing the inner workings of a particle, such as a living cell, preferably without any dyes, labels or other markers. The xe2x80x9cOptophoretic Constantxe2x80x9d of a cell uniquely reflects the physiological state of the cell at the exact moment in which it is being analyzed, and permits investigation of the inner workings of cells. These techniques allow simple and efficient gathering of a wide spectrum of information, from screening new drugs, to studying the expression of novel genes, to creating new diagnostic products, and even to monitoring cancer patients. This technology permits the simultaneous analysis and isolation of specific cells based on this unique optophoretic parameter. Stated otherwise, this technology is capable of simultaneously analyzing and isolating specific particles, e.g. cells, based on their differences at the atomic level. Used alone or in combination with modern molecular techniques, the technology provides a useful way to link the intricate mechanisms involving the living cell""s overall activity with uniquely identifiable parameters.
In one aspect, the invention is a method for the characterization of a particle by the steps of observing a first physical position of a particle, optically illuminating the particle to subject it to an optical force, observing the second physical position of the particle, and characterizing the particle based at least in part upon reaction of the particle to the optical force. The characterization may be that the particle, e.g., a cell, has a certain disease state based upon the detected optophoretic constant or signature.
While characterization may be done with or without physical separation of multiple particles, a method for separating particles may consist of, first, subjecting particles to optical gradient force, second, moving the particle, and third, separating desired particle from other particles. The particle may be separate from the others by further optical forces, by fluidic forces, by electromagnetic forces or any other force sufficient to cause the required separation. Separation may include segregation and sorting of particles.
In yet another aspect, the invention includes a method for analyzing particles by electrokinetically moving the particles, and subjecting the particles to optical forces for sorting. The electrokinetic forces may include, for example, eletroosmosis, electrophoresis and dielectrophoresis.
In addition to the use of the dielectric aspects of the particle for characterization and sorting, certain of the inventive methods may be used to determine the dielectric constant of a particle. One method consists of subjecting the particle to an optical gradient force in a plurality of media having different dielectric constants, monitoring the motion of the particle when subject to the optical gradient force in the various media, and determining the dielectric constant of the particle based upon the relative amount of motion in the various media.
Yet other methods permit the sorting of particles according to their size. One method includes the steps of subjecting the particles to a optical fringe pattern, moving the fringes relative to the particles, wherein the improvement comprises selecting the period of the fringes to have a differential effect on differently sized particles. An allied method sorts or otherwise separates particles based upon the particles flexibility when subject to a optical force. One set of exemplary steps includes: subjecting the particles to an optical pattern having fringes, the fringe spacing being less than the size of the particle in an uncompressed state, moving the fringes relative to the medium containing the particles, and whereby particles having relatively higher flexibility are separated from those with relatively lower flexibility.
In addition to the use of optical gradient forces, the systems and methods may use, either alone or in combination with other forces, the optical scattering force. One method for separation in an optophoresis set up consists of providing one or more particles, subjecting the particles to light so as to cause a scattering force on the particles, and separating the particles based upon the reaction to at least the scattering force.
Various techniques are described for enhancing the sensitivity and discrimination of the system. For example, a sensitive arrangement may be provided by separating the particles in a medium having a dielectric constant chosen to enhance the sensitivity of the discrimination between the particles, and changing the medium to one having a dielectric constant which causes faster separation between the particles. One option for enhancing the sensitivity is to choose the dielectric constant of the medium to be close to the dielectric constant of the particles.
Accordingly, it is an object of this invention to provide a method of identification, characterization, selection and/or sorting of materials having an optical dielectric constant.
It is yet a further object of this invention to provide a system for sorting or identifying particles without labeling or otherwise modifying the particle.
It is yet another object of this invention to provide a system in which uncharged or neutral particles may be sorted or otherwise characterized.
Yet another object of this invention is to provide a system in which particles may be manipulated remotely, thereby reducing the contamination to the system under study.
It is yet another object of this invention to provide a system for characterizing, moving and/or sorting particles that may be used in conjunction with other forces, without interference between the optical forces and the other forces.